The invention is an automatic pressure control valve which can be used for pressure control of a liquid or gas media flow. It can be used to control the gauge pressure, or the differential pressure between two points in a system.
It can control the pressure in for example; compressed air, water or steam lines, oil and fuel supplies and air handling systems. It can also be used to control the liquid level in tanks.
An important application is together with control valves, for automatic flow rate control. The automatic pressure control valve is piped in series with the control valve and arranged so it maintains a constant differential pressure across the control valve. The two valves works together as a pressure independent control valve.
Automatic pressure control valves are used to control liquid or gas media flow so the pressure is essentially constant. A common type of automatic pressure control valve has the controlled media acting against one side of a diaphragm, and the opposite side is connected to the atmosphere. The differential pressure over the surface over the diaphragm produces a force which is opposed by a spring. Typical examples are shown in U.S. Pat. Nos. 4,044,792 and 5,009,245.
Pressure variations causes unbalance between the two forces and produces a net force that moves the diaphragm. The diaphragm operates a valve mechanism, which increases or decreases the pressure of the media until there is balance between the diaphragm and the spring. Thus, the spring tension determines the set-point of the automatic pressure control valve. In the following text the automatic pressure control valve is referred to as APCV.
The diaphragm is connected to the valve mechanism via a stem that passes through a packing or similar. Its friction together with the friction of the valve mechanism must be overcome by the net force from the diaphragm and spring.
In order to get a smooth control, with only a small hysteresis, the diaphragm and spring need to be relatively large so already a very small pressure variation produces a net force strong enough to overcome the friction.
This is not the only reason for using a large diaphragm. The control surface of the valve mechanism has an area against which the media pressure acts and produces a force. This force typically counteracts the spring force. Therefore, variations in the media pressure will change the set-point. This is especially true if the control mechanism must have a large flow capacity.
When a large flow capacity is needed, the control surface must be large, and the force counteracting the spring is quite large. Therefore the diaphragm and spring need to be large so the influence of the media pressure on the set-point will not be too large.
Instead of increasing the area of the control surface its movement can be increased. This will also increase the flow capacity. The drawback is that the movement changes the spring tension, which also changes the set-point. The change depends upon how much the control surface needs to open, which is a function based upon both the flow and the pressure.
The change in set-point is reduced by using a long spring, so the movement is small compared to the length of the spring. However, this increases the size and cost of the APCV.
The above described APCV balances the controlled pressure against the atmosphere and a spring. Many other types of valves exists. For example, instead of connecting to the atmosphere, both sides of the diaphragm are connected to the media, but to different points of the system. A built in spring acts against the diaphragm and the valve mechanism regulates the media flow so a controlled differential pressure is maintained between the two points. This is an automatic differential pressure control valve. In the following text referred to as ADPCV.
From the above it is understood that in order to achieve a good accuracy APCVs ADPCV""s need large diaphragms and springs. This of course means that also the housing surrounding the diaphragm need to be quite large and costly.
There are some valve mechanisms that have not the above described problems. However, many of these valves (sleeve type, for example) tend to leak, so the very small flows can not be controlled.
It is also possible to use pilot valves to operate the diaphragm to improve the accuracy. However, this an added complication which increases the cost.
The above is a brief summary of the some of the problem associated with APCVs.
Automatic control valves in HVAC and industrial process applications are fitted with actuators that operates the control valves in response to signals from controllers, so the correct flow is provided. The problem is that the flow not only depends upon how much the valves are open, but also upon the differential pressure across the valve.
The differential pressure depends upon the operating conditions of the whole piping system.
A sudden pressure variation in the piping system changes the flow through a control valve and the control is upset. It takes some time before the control system signals the actuator to change the opening of the valve so the correct flow is obtained and stable control is restored.
Control valves are made with a certain flow characteristics, which defines how the flow changes as the valve opens.
The flow characteristics is designed with a curvature that compensates for the non-linear characteristics of the control object (often heat transfer devices). The objective is that the total characteristics is linear, from the signal to the actuator to the output of the control object. This is very beneficial for stable control.
The flow characteristics of a valve is laboratory tested at a constant differential pressure.
Pressure variations due to load changes distorts the flow characteristics of the control valves, which is detrimental for stable control.
It is very difficult to correctly size control valves. The flow coefficient needs to be calculated. It is calculated by multiplying the flow rate (GPM) by the square root of the specific gravity of the liquid and then divide by the square root of the differential pressure at the maximum load conditions. Unfortunately, it is very difficult to obtain a correct information about the differential pressure that reflects the actual conditions. One of the reasons is that the xe2x80x9cas built conditionsxe2x80x9d deviate form the specification.
Without correct information, the control valves will not be sized correctly. Undersized control valves can not supply the needed flow and must be replaced. To avoid this the tendency is to install oversized control valves. However, it is very detrimental for stable control, especially at low loads.
The problem can be solved by combining the control valve with an ADPCV, and arrange it so it maintains a constant differential pressure across the control valve.
With a constant differential pressure across the control valve a well defined flow rate is provided for each degree of opening of the control valve. The flow rate is independent of pressure variations in the piping system before and after the valve combination. Therefore, the combination of an ADPCV and a control valve is referred to as a PRESSURE INDEPENDENT CONTROL VALVE (in the following text called PICV).
Because of the constant differential pressure the control valve will always operate with a perfect valve authority and therefore the flow characteristics will not be distorted by pressure variations in the piping system.
The PICV can be applied in different ways.
It can be used as an automatic flow rate controller, with a manually adjusted set-point, and can have a handle and a graduated indicator disk to adjust the flow rate. Applications are where a constant, or manually adjusted flow rate is needed. It can also be used as a high limit in applications with a variable flow
The PICV can be operated by an actuator, which responds to signals from a controller.
The maximum flow through the PICV can be set by limiting the maximum opening of the control valve. This can be done by limiting the stroke of the actuator.
The PICV can provide significant improvement of the quality of control in an industrial process or in a HVAC control system. The problem so far has been the high cost.
It is primarily the ADPCV that increases the cost. The reason is the relatively large diaphragm, spring and housing. Typical examples of PICV""s are shown in U.S. Pat. Nos. 5,143,116 and 5,775,369.
The invention is a simple APCV, ADPCV and PICV.
The APCV are referenced to the atmosphere and controls the gauge pressure. There are two types.
The first type of APCV controls the downstream pressure.
The second type of APCV controls the upstream pressure.
APCV""s for general pressure control applications are shown.
Special APCV""s for level control in tanks are also shown.
The ADPCV""s are the same as the APCV""s, the difference is that they are not referenced to the atmosphere. They are instead referenced to a second point in the fluid flow system and controls the differential pressure.
The PICV""s are control valves connected in series with ADPCV""s and controls the fluid flow rate through the valve independently of variations in the line pressure.
The first type of APCV has a body with a passage way for a fluid flow between an inlet and an outlet. Intersecting the passage way is a seat, against which a control disk operates and regulates the flow. The control disk is on the downstream side of the seat and controls the outlet pressure. On the upstream side of the seat there is a diaphragm from which the control disk is suspended by a stem. The effective surface area of the diaphragm and the control disk are the same, so the two are balanced. Instead of a diaphragm, a piston, disk bellow or any other suitable pressure sensing device can be used.
The incoming pressure acts upon the underside of the diaphragm and the top side of the control disk. The forces are equal and acts in opposite directions so they neutralize each other.
The cross section area of the connecting stem is not important for the balance of forces, because it affects the bottom side of the diaphragm and the top side of the control disk equally.
The only difference is that a stem with a large cross section area results in a smaller opposing forces than a small diameter stem. Either way the opposing forces are equal and neutralize each other.
The pressure differential between the top side of the diaphragm and the bottom side of the control disk, acts over the effective area and produces a force. Under normal conditions, the pressure above the diaphragm is less than the pressure under the control disk. Thereby, the net force is directed upward and strives to move the control disk up against the seat. A spring provides an opposing force and pushes against the top of the diaphragm.
If the outlet pressure for any reason increases, the control disk moves towards the seat and reduces the annular opening between the two. This increases the flow resistance and which reduces the pressure under the control disk. Automatically, the control disk moves and adjusts the annular opening so the outlet pressure assumes a value that produces a balance of forces.
The spring tension determines the set-point for the pressure (Instead of a spring or in combination with, an air pressure can be used, or weight, or magnet, proportional solenoid, or similar.)
When the top side of the diaphragm is connected (referenced) to the atmosphere the APCV will control a gauge pressure. The spring tension divided by the effective area equals the set-point.
In the following text the xe2x80x9ceffective areaxe2x80x9d refer to the side of the pressure sensing member facing the reference pressure, or the side of the disk or cup facing the controlled pressure.
The second type of APCV is similar to the first type, with the following exceptions.
The control disk is on the downstream side of the seat and controls the inlet pressure. On the downstream side of the seat there is a diaphragm to which the control disk is connected by a stem.
The outlet pressure acts upon the underside of the diaphragm and the top side of the control disk. The forces are equal and acts in opposite directions so they neutralize each other.
The pressure differential between the top side of the diaphragm and the bottom side of the control disk, acts over the effective area and produces a force. Under normal conditions, the pressure above the diaphragm is higher than the pressure under the control disk. Thereby, the net force is directed downward and strives to move the control disk down against the seat. A spring under the control disk, opposes the force. (In the case where the pressure above the diaphragm is less than the pressure under the control disk, the spring instead can be located above the diaphragm against which it pushes down.)
The control disk moves down against the seat if the inlet pressure increases. This increases the flow resistance and the pressure under the control disk. It automatically finds the position that produces the inlet pressure that produces a force that balances the pressure above the diaphragm minus (or plus) the spring force.
The spring tension determines the set-point for the pressure. (Instead of a spring or in combination with, an air pressure can be used, or weight, or magnet, proportional solenoid, or similar.)
When the top side of the diaphragm is connected (referenced) to the atmosphere the APCV will control a gauge pressure.
When the top side of the diaphragm is connected to second point in a system, the ADPCV will control a differential pressure between the outlet of the automatic pressure control valve and the second point. The second point should be downstream of the ADPCV.
It is very important that the effective areas of the disk and the diaphragm are essentially the same. Otherwise, the pressure will not be controlled at a stable value.
If the effective areas are different, APCV""s controlling the outlet pressure will be affected by the inlet pressure, and APCV""s controlling the inlet pressure will be affected by the outlet pressure.
In order to get a well defined effective area of the disk, its perimeter should have a pointed edge which makes contact with the seat at a specific diameter. The pointed edge is also needed to get a high contact pressure against the seat, so a tight close-off can be accomplished. It is also advantageous if the seat is conical so the disk will self-center.
To maintain the same pressure across the surface of the top of the disk, the diameter of the disk should be only slightly larger than the inlet opening of the conical seat. Otherwise the pressure near the perimeter of the disk may drop at higher flow rates. The force balance will be changed and the controlled pressure will drop noticeably at high flow rates.
The PICV""s are control valves connected in series with ADPCV""s, which are arranged to control the differential pressure across the control valve. Thereby, fluid flow rate through the valve is determined only by the degree of opening of the control valve, and is independent of variations in the line pressure.
The differential pressure is picked up from the second point and can be communicated to the diaphragm of the ADPCV via an external pipe. However, when the ADPCV and control valve are one unit, it is advantageous to use an internal connection.
When a PICV uses the same body for the control valve and the ADPCV, a channel can be made inside the body to communicate the differential pressure of the control valve to the diaphragm of the ADPCV.
If the control valve (of the PICV) is a globe valve, curtain or a top entry ball valve, the main body can be in one piece. The differential pressure is then connected from just after the control valve, and it is relatively easy to make the channel.
If the control valve is a xe2x80x9ctwo piecexe2x80x9d ball valve, having a main body and a nipple, the differential pressure should not be connected from the nipple, because it is impractical to line up a channel from the nipple to the main body. Instead, the ball should have a small hole from the bore through the ball, to the cavity surrounding the ball. The cavity is connected via a channel to the diaphragm of the ADPCV. Thus, the pressure inside the ball is communicated to the diaphragm. An added advantage is that when the ball is in the closed position, communication between the diaphragm and the outlet of the PICV is closed. When servicing the APCV only a upstream shut-off valve needs to be closed.
The internal parts (diaphragm with its chamber and spring, shaft, seat and disk) of the APCV or ADPCV can be built as one unit, in the form of an insert. The insert fits into a special recess in the body. This simplifies service and replacement of the APCV.